You may not have a backflow preventer in the main line. This prevents anything coming back from the main line. It may be coming from your neighbors above you on the line.
Any thing from hair to toys to roots or anything you jam down your toilet or drain pipe that wont decompose. I have snaked out a lot of sewer lines and I have to say that the number one reason for a...
Roots, rags hard stoppages require rodding (snakes) soft stoppages grease ,soil require the use of water Jetters to scour the lines to restore full flow